1. Field of the Invention
The present invention is directed to a method and a system for transmitting data in the gigabit/sec range in a galvanically isolated manner, such as via a slip ring arrangement, particularly between the rotating gantry and the stationary data processing arrangement of a computed tomography apparatus.
2. Description of the Prior Art
Medical imaging devices, such as computed tomography systems of the third generation, use slip rings to transfer the measurement data from the data measurement system (DMS) which is located on the continuously rotating gantry to the stationary imagery construction system (IRS). The DMS must be electrically isolated from the rotating gantry to avoid leakage currents and electromagnetic interference from perturbing the noise-sensitive measurement channels of the DMS. Additionally, the IRS should be electrically isolated from the gantry so as to avoid electrical hazards to the operator with respect to the high voltages that are employed to operate components on the gantry, as well as to avoid electromagnetic interference produced by the gantry from disturbing the IRS, and electromagnetic interference produced by the IRS from disturbing the gantry components.
Newer computed tomography systems employ a two-dimensional detector array in the DMS, and thus are able to measure more slices per rotation. As an example, the Siemens Somatom Volume Zoom last generation CT scanner acquires four slices simultaneously, thereby producing measurement data at a rate of about 200 Mbit/sec. New clinical applications an even further increase in the number of simultaneously measured slices, which increases the data rate which must be transferred from the DMS to the IRS into the gigabit/sec range.
Transferring data serially in the gigabit/sec range from the data source to the data destination is particularly difficult when using successively connected (chained) communication links, because of jitter accumulation. Jitter is a generic term for the effect of random or deterministic shifts of one to zero transitions and zero to one transitions in the binary data within a serial bit stream that are not quite in phase with the reference clock. These shifts are visible as deviations from the mean phase which occurs in the so called eye diagram. Within a data transmission link, each component transfers the jitter received at its input and adds further, internally generated jitter at its output. The quality of a data link can be assessed by the amount of jitter which occurs at the output end of the link, because it is at this location where a clock and data recovery (CDR) circuit must correctly decode the bits. In spite of jitter, the CDR circuit must provide a satisfactory bit error rate (BER). In addition to jitter, noise injected into the link and pulse distortions due to non-linery components further degrade the overall BER. ITU-T recommendations specify the limits for jitter transfer, jitter generation and jitter tolerance (see ITU-R BT.1363, “Jitter Specifications and Method for Jitter Measurements of Bit-serial Signals”).
In a computed tomography system, usually the data source includes an optical transmitter that emits the measurement data serially to the slip ring system (SRS), where a rotary reception module converts the incoming optical bit stream into an electrical signal. The electrical signal is transmitted to the stationary SRS module, either by capacitive coupling or optically using a high-power laser. The stationary SRS module converts the signal received from the rotary part into an electrical bit stream and transmits it further to the receiver via an optical transmitter to the data consumer (destination). Such a known arrangement is schematically shown in FIG. 1 herein.
Therefore, the SRS functions as a transmission channel that is transparent to high-level protocol that behaves as a logical segment of the communication link. In order for the link to function, it must be insured that the jitter contribution of all components between the sender and receiver is sufficiently low so that the receiver is still able to decode the incoming bit stream with a reasonable BER. This requirement can be achieved in a satisfactory manner for data rates below 200 Mbit/sec using standard components.
For gigabit links (>1,000 Mbit/sec), IEEE standards specify the communication link jitter budget and how it is distributed along the link components in IEEE Draft P802.3z/D5.0, May 6, 1998, “Supplement to Carrier Sense Multiple Access with Collision Detection (Csma/cd) Access Method & Physical Layer Specifications-Media Access Control (Mac) Parameters, Physical Layer, Repeater and Management Parameters for 1,000 Mb/s Operation.” Section 38.5 of this specification lists the jitter margins as well as the required measurement set-up. As shown in FIG. 2 herein, this standard defines four compliant points TP1, TP2, TP3 and TP4 at which jitter is specified, and three link segments that may add additional jitter, namely TP1 to TP2, TP2 to TP3 and TP3 to TP4.
The jitter budget is distributed according to the following table:
Compliance PointTotal jitter [ps]TP1192TP1 to TP2227TP2345TP2 to TP3136TP3408TP3 to TP4266TP4599Numbers in the above table represent a high-frequency jitter (above 500 kHz) and do not include low frequency jitter or wander that occurs inherently due to rotation in the case of slip ring transmission.
FIG. 3 herein shows the gigabit link decomposition from the point of view of jitter, when using a slip ring system between the sender and the receiver, and the available and realizable jitter budget. As shown in FIG. 3, additional jitter-producing components exist by virtue of the inclusion of the slip ring system (SRS) between the standard compliance points TP2 and TP3. The optical components of the SRS modules exhibit a deterministic jitter, for which typical figures are shown in FIG. 3. Moreover, the slip ring internal transmission contributes additional jitter. For example, a capacitive slip ring set-up operating experimentally at 400 MHz without rotation, and outside of the usually noisy environment of a CT gantry, exhibited about 500 ps total jitter only for the electrical transmission itself. According to these values, the SRS generates overall approximately 600 ps jitter, which is far beyond the allowable limit of 136 ps. Therefore, data transmission in the gigabit/sec range via an isolated slip ring arrangement using standard components and known techniques is unsuitable, due to the excess jitter. Even attempting to employ extremely fast optical components to reduce the jitter would not overcome this problem, because the overall link, due to the presence of the capacitive slip-ring transmission arrangement, would still be subject to additional externally injected noise.